During the last century practically all freight trains have used a standard handbrake operation to keep unattended vehicles or freight cars from rolling away. By means of a wheel or handle mechanical force has been applied against the brake shoes to develop enough friction to prevent the wheels from rotating. This practice applies to individual and groups of coupled vehicles and vehicles forming part of a train consist.
When a train consist stops, the pneumatic brake will produce braking force which holds the vehicles stationary only so long as compressed air is available. During long periods the air will leak out and the braking pressure will drop so low as to be inadequate to brake the vehicles. When that happens, or may likely occur, a trainman must walk the length of a train, many of which are a mile long, and set hand brakes to park the vehicles indefinitely. He must then walk the train again to release the handbrakes before the train can be moved. Because of train lengths this takes considerable time. As a result, it has become common practice to set only one handbrake out of several to reduce the tiresome labor and time involved in this task. What happens though is that the same or a different trainman later cannot readily tell which brakes are set and must be released so that much too often brakes are not released when vehicles and trains are moved again. The brake resistance can cause wheels to overheat and cause wrecks. Also, because the handbrakes are in accessible to the crew while a freight train is moving, they are useless as a fail safe device in the event all air pressure is lost so the handbrakes add nothing to safety during vehicle movement but can only lower safety.
An economic evaluation by the Association of American Railroads has determined that a significant portion of the yearly wheel replacement cost of $40,000,000 is attributable to handbrake problems and this apparently does not include accident related cost or take into consideration the personal hazard involved. Also, the expense of manually operating the handbrakes independently is very high.
The described braking problem is not limited to conventional freight trains. In recent years there has been considerable interest in the United States in transportation equipment which is suitable for both railroad and highway use. There is particular concern with developing equipment which will permit over the highway trailers to be coupled together to form railroad trains of such trailers. In one system, a railroad truck is releasably coupled through an adapter to the rear bottom of each trailer. The front end of a following trailer is also coupled to the rear end of a leading trailer by a male-female coupling system. One system of this type is disclosed in Wicks et al U.S. Pat. No. 4,669,391. After a train of trailers coupled together in this manner reaches its destination the trailers are uncoupled from each other and from the railroad trucks, which remain on the tracks. The trailer wheels are lowered and then the trailer is towed away by a tractor. The railroad trucks must then be stored until needed to form another train of over the highway trailers. For safety, each truck should have its own handbrake so that it can be prevented from rolling away.
Each railroad truck used in trains of the described type are desirably provided with all or nearly all the braking apparatus necessary for required braking, other than the service braking air pressure provided by the train through an air line carried by each trailer for the train length. The brakes may be of the type shown in Mersereau U.S. Pat. No. 3,442,358. While the braking system disclosed in this patent utilizes a truck mounted cylinder for braking operation, it appears that it must use only conventional service braking air pressure and conventional emergency braking air pressure supplied from a reservoir mounted on a car body. A need exists, accordingly, for a largely truck mounted braking system which incorporates a single dual acting cylinder which controls a plurality of desirable braking functions including service braking, fail safe emergency braking, automatic parking braking, automatic air release of parking braking, parking brake manual release and variable load or empty load braking.
From the above discussion it is clear that a need exists for improved parking brakes on conventional freight cars, trains of trailers and railroad trucks which improve safety, reduce labor and improve overall reliability of the braking system.